The present invention relates to multilayer, stripline and microstrip circuit board construction and more particularly to microwave antenna construction.
Multilayer stripline circuits have heretofore been fabricated using 4 to 6 substrates to produce 2 or 3 circuit trace layers (not including ground planes). These stripline and/or microstrip devices sometimes require multiple bonding cycles to make the completed assembly. Once the substrates have been bonded together, via holes are drilled completely through the bonded assembly and plated for transferring electrical connections between different layers. This limits the circuit density and number of substrates. Generally, holes have not been drilled or plated prior to bonding because they tend to break under the high bonding temperatures and pressures encountered. The bonding material, which may comprise a bonding film between the substrates, tends to flow out of the assembly under the influence of the high temperature and pressure. As more and more of the bonding material flows out, the pressure is transferred to the plated holes which may collapse and/or be pushed into the substrate which has been softened by the high temperature. The substrates for microwave circuits are usually thicker than for other type circuit board constructions and are comprised of Polyterafluoroethylene (PTFE) or a PTFE-based material. PTFE, however, has a high coefficient of thermal expansion and thus tends to soften at high temperatures. This results in alignment problems, distortions in the final assembly, and stress at the plated holes. Another problem is that most of the internal connections produced are not normally impedance matched, thereby decreasing the electrical efficiency.
The industry is currently trying to find methods of producing multilayer microwave circuit boards to eliminate these problems. One such effort is reported in "Microstrip Antenna Manufacturing Technology Program--Phase II Report" by Boeing Aerospace Company, Seattle, WA. This report describes methods of assembling a microstrip antenna using pure silver as a conductive bonding material and therefore requires very high temperatures and pressures to bond the circuit boards together. Such temperatures and pressures are a marked disadvantage in the production of these type microwave devices.
We believe that the present invention eliminates and/or reduces the problems that have affected the prior art as it pertains to microwave antenna construction methods and allows multiple circuit boards to be integrally bonded in a single bonding cycle using lower temperatures and pressures than herebefore used.